Anal. Chem. 1982, 54, 2333-2336
2333
Uranyl Selective Electrode Based on Organophosphorus Compounds Chlen-Shu Luo, Fu-Chung Chang, * and Yu-Chal Yeh Institute of Nuclear Energy Research, Lungtan, Taiwan 325, Republic of China
A comparatlve study on the polentlal functions of different membrane components showed that the comblnation of trln-butyl phosphate, trl-n-octylphosphlne oxide, and sodlum tetraphenylborate In the PVC matrix gave the best full Nernstlan response (59 mV/decade) and It gave nearly the same potential behavior among uranyl chiorlde, nitrate, and sulfate bathing solutions. Due to the high afflnlty with uranyl ions, the membrane containing trl-noctylphosphine oxlde can be responsive to both catlons and anlons dependlng on the concentratlons of the bathing solutlons. Incorporatlon of sodium tetraphenyiborate was found necessary for obtalnlng permselectlvlty of the cations. Monovalent cations were regarded as permeating species for the membrane.
Uranyl ion sensors employing organophosphorus compounds as ion exchangers and mediators have been reported (1-4). However, there exist noticeable discrepancies on the use of bis(2-ethylhexy1)phosphoric acid (DEHPA) which had been intensively studied and considered as the more favorable one from the earlier works. The reported values of the potential responses of such a DEHPA based membrane electrode could be found from 35 t o 20 mV/decade (1-4). Moreover, the membrane in the bathing solutions of different uranyl salts behaved differently. The abnormal anionic response was described particularly in uranyl nitrate solutions ( 4 ) . Manning also noted the interferences of nitrate and perchlorate anions for the electrode in uranyl chloride solutions (1). Obviously the suitable organophosphorus compounds which would give favorable potential response, give high selectivity, and be applicable to the uranyl chloride, nitrate and sulfate bathing solutions were not observed conclusively. Use of the weak ligandlike phosphites as ion exchangers (3) may reduce the selectivity due to obtaining the lower value of partition coefficients of the interesting cations and thus probably increase the value of the exchange equilibrium constants for most of the foreign ions (5-7). The present work introduces the strong ligands tri-noctylphosphine oxide (TtOPO) for the uranyl ion electrodes and compares the potential functions of the membranes with different compositions in order to find the optimum conditions for the construction of a uranyl ion selective electrode, which is applicable among varlous uranyl bathing solutions.
EXPERIMENTAL SECTION Membrane Preparation. The PVC matrix membranes were prepared as follow!3: In a 20-mL weighing bottle, the diluent tri-n-butyl phosphate was added with various amounts of the ligand compounds chosen as ion exchangers of the membrane electrodes. The ligands TOPO and DEHPA were examined and the amounts were in the range of 20-200 mg. In some cases sodium tetraphenylborate was also added to the mixture in order to improve the permselectivity of the cations. To the resulting mixture was then introduced 5.25 g of PVC solution, which was prepared by dissolving 2.75 g of PVC powder in 60 mL of tetrahydrofuran. After complete dissolution of all components and
thorough mixing, the final solution was divided in two approximately equal portions and each poured into two glass rings which were placed on a smooth glass plate. The glass rings had an inside diameter of 26 mm. They were then covered with filter papers on which a weight was placed for about 48 h to allow the evaporation of the solvent. The transparent membranes could be finally obtained with an average thickness of 0.3 mm. A 5.7 mm diameter piece was cut out with a cork borer and glued to the polished end of a PVC tube by means of the PVC solution already mentioned for the construction of the electrode. Potential Measurement. The electrochemical system was as follows: AglAgC1, satd KClltesting solutionlmembranelinternal filling solution (pH 3)lsatd KC1, AgCliAg. The internal filling solution was the same uranyl salt as the testing solution, and their concentrations ranged from to M. Such a cell permitted examination of the effects caused by different anions for a variety of uranyl salts. The cell potential was measured with an Orien Research electrometer, Model 801A. The testing solutions included uranyl chloride, nitrate, and sulfate in the concentration range 1.0 X lo4 to 1.0 X 10-1 M. Except for chloride solutions, uranyl nitrate and sulfate solutions were prepared from reagent grade chemicals without further treatment. The series solutions were obtained by appropriate dilution and brought to pH 3.0 by using their corresponding dilute acid or NaOH. Uranyl chloride was prepared from the nitrate salt according to Manning ( 1 ) . Unless otherwise stated, potential was measured in constantly stirring solution and a reading was taken at 2 min after immersion, or 5 min after immersion for concentrations less than lo4 M. Each new membrane electrode was left standing overnight after being fiied with internal solution to carry out the process of ion exchange of the counterions for the membrane. The electrode was soaked in M uranyl solution for 1h prior to daily measurements and stored in air when not in use. Evaluation of Electrode Performance. The selectivity coefficient was evaluated based on the potential measurements in mixed solutions (8, 9) which contained constant uranyl concentration and varied amounts of interfering ions. The selectivity coefficient was graphically determined according to the equation
where KMNPot is the selectivity coefficient, El and E’ are the potentials measured for pure uranyl solution of activity aM and for solution mixed with interfering ions N”+, respectively, a‘M is the activity of uranyl ions in mixed solution which contains interfering ions of activity a“, and z and n are the charges of counterions and interfering ions. The activity coefficient of the uranyl ions was estimated similarly to the work of ref 10. The activity of UOZ2+was obtained as the product of the activity coefficient and the calculated concentration of free U022+at pH 3.0. The liquid junction potential between saturated KCl and testing solution was computed by use of the Henderson equation ( 1 1 ) . The calculations indicated that the difference of liquid junction potential resulting from the decade change in ionic activity of the solution was less than 2.6 mV and hence was negligible.
RESULTS AND DISCUSSION The behavior of various membranes with DEHPA or TOPO as ion exchangers and T B P as solvent mediator was summarized in Table I. T B P was used in all cases in view of the fact that a suitable solvent mediator had t o be employed in
0003-2700/82/0354-2333$01.25/00 1982 American Chemical Society
2334
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
Table I. Composition and Response Characteristics of PVC Matrix Membranes composition of membrane
membrane
TBP, mg
DEHPA,
TOPO,
mg
mg
1 2 3 4 5 6
400 400 400 400 400 400
7
400
20
8 9 10
400 400 400
150
11
400
12
400
13
400
20 150
sodium tetraphenylborate, mg
potential response slope per linear detection decade, mV region UOz2+,M 36 29
10-3-10-1 5 x 10-4-10-2 a , b
erratic
20 50 100
31 46 36
20 200
anionic 118 erratic anionic 8 1
100 100
20 50
50
50 50 100
10-5-10-3 a 10-5-10-2 a 10-5-10-3 a 10-2-10-1 b 2 x 10-2-10-1 b 5 x 1 0 - ~ - 1 0 -b ~ 10-4-10-1 a,c 1 0 - ~ - 2x io-2 b 5 x 1 0 - ~ - 1 0 -b~ 10-2-10-1 a
34 59
59
a
UO,SO, bathing solution.
U0,(N03), bathing solution.
conjunction with a PVC electrode membrane (1, 12). T B P not only dissolved the exchanger salt but also is regarded as a solvating solvent (13-15) which probably enhanced the membrane selectivity by partly solvating the uranyl ions within the membrane and consequently influencing the formation constants and partition coefficients (12). The membrane containing T B P alone (membrane 1) was also responsive. The potential response from membranes 2 and 3 containing DEHPA along with T B P showed an insufficient reproducibility, and a large variance of the potential measurements was obtained. The one with high content of DEHPA (membrane 3) even gave an erratic response which could not be considered a functional electrode. Moreover, a prominent “memory effect” was also observed for the DEHPA based membrane electrode. These may be reasons why discordant results were found in the literature for the DEHPA based uranyl electrodes (1-4). The feature of the preparation of the membrane in this study was that the addition of uranyl organic exchanger was eliminated. Instead, uranyl counterions were built into the membrane by the later soaking and ion exchanging process. The question now arises as to whether introduction of the prepared uranyl organophosphorus complex rather than the direct incorporation of DEHPA in the PVC membrane will result in a favorable electrode. Alternatively, a preliminary study employing prepared uranyl bis(2-ethylhexy1)phosphateion exchanger according to ref 1 and using T B P as diluent attained the membrane with slope 28 f 10 mV in uranyl chloride bath solutions for nine individual determinations. The variance of the slope value was attributed to the noticeable memory effect on repeated potential measurements over the uranyl concentration range as well as the instability of the membrane potential in uranyl solution. Therefore, the DEHPA based uranyl membrane electrode with PVC matrix seemed to have little practicable value in this regard. Recall the results dealing with the extraction of uranium with DEHPA. This ligand exists predominantly in dimeric form in most of the solvents, and the extraction reactions for uranium(V1) are seldom stoichiometrically equivalent. Formations of the uranium-DEHPA species like UOzXzH2and (UOz)z&Hz (16-18), X = (R0)2P02-,have been reported from extractions a t low acidic media and low uranium levels. At high uranium levels, other species are formed and leading to a limiting polymeric chain (U02)nX2n+2H2 with a uraniumDEHPA ratio of 1:2 (19,20). As a consequence, the equilibrium constant and the partition coefficient for the uranium-
51 59 59
10-2-10-1 a
W02C12 bathing solution.
;j
E
I
LL
I W
! -5.0
-4.0
-3.0
-2.0
-1.0
l o g auOz
Flgure 1. The potential responses of the membranes containing TBP TOPO: (a) 20 mg of TOPO, (b) 50 mg of TOPO, (c, d) 100 mg of
+
TOPO;
(a-c) UO,SO, bathing solution, (d) UO,(NO,),
bathing solution.
DEHPA extraction system are not expected to remain the same, as the extent of uranium loadings in the organic phase is greatly changed. Accordingly, such an effect would give rise to the deviations of the electrode potential (21). Furthermore, once the DEHPA based membrane was saturated toward high uranium loadings, the highly viscous chain polymer appeared difficult to reequilibrate in the more dilute aqueous solutions due to the hindered diffusion. Pronounced memory effect and erratic responses were hence obtained for this type of membrane electrode. Membranes based on TOPO showed (see Figure 1)either cationic or anionic responses depending on the amount of TOPO impregnated and the concentration of the uranyl solution in which the membranes were exposed. The phosphine oxides were strong solvating solvents because of their higher basicity. TOPO formed more definite solvates and bind more strongly to acids (15). The species extracted from uranyl chloride and nitrate solutions were UOzCl2-2TOPO and U02(N03)2.2TOP0. The stoichiometric saturation could readily be obtained (22). Failure of coion exclusion was noted for the TOPO based membranes, especially when large
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
Table 11. Correlation between the Affinity of the Ligands and the Electrode Response Characteristics
ligand triphenyl phosphate tris( dimethylphenyl) phosphate diphenylmethyl phosphate tributyl phosphate bis( 2-ethylhexyl) 2-ethylhexylphosphonate dibutyl butylphosphonate trioctylphosphine oxide
partition coefficients of U( VI) in low acidic media 0.00067n
idi,
cationic slope, mV/ deacde 12c 17c 14
0.25a
12,c 36d 1oc
10.0a 7,OOb
1oc
7d
(nitrate salt.) 1.0b
36d
-50
(sulfate salt) a Data from ref 23. ref 3. This study.
2335
Data from ref 22.
Data from
amounts were incorporated (Figure Ic,d). For uranyl nitrate solutions, such a membrane was nearly responsive to the nitrate ions, as a negative slope of 118 m'V/decade was obtained. Whereas for uranyl sulfate solutions in which the effectiveness of extraction gave a lesser extent (22), the membrane was responsive to the cations a t the low sample activities and also to the anions at high activities. A maximum response was observed (Figure IC).And again the negative slope of 55 mV/decade in this case presumed the sulfate response as noted with that of the nitrate stated above. The cation response was sub- Nernstian in all TOPO based cases (membranes 4-6, Table I). Decreasing the amount of TOPO in the membrane shifted the maximum response to higher sample activities. Combinations of the ion exchangers of TOPO and DEHPA did not give good results (membrane 7 , 8). In a survey of the theoretical treatments on the liquid membrane electrode without charged ion exchange sites (24-26), the following parameters were of concern: (1)the permeability of the individual ions (cations, anions, and their complexed forms), (2) the relative mobility of each ionic species within the membrane, (3) the affinity of the complexing ligand to the objective ions, (4) the extent of the ion pair formation between the complexed cations and the anions, (5) the degree of ligand association with cations at the membrane surface. It is known that the relative order of the extraction strength of uranium by the phosphoric acid derivatives is phosphine oxide > phosphinate > phosphonate > phosphate. Table I1 exhibits the correlation between the affinity of the ligands and the electrode response characteristics. Membranes with phosphate ligand generally showed better slope response than those of phosphonate, while the latter gave relatively stronger ability for the uranium extraction than the former. Membranes containing phosphine oxide with sample solution of uranium nitrate in which the ligand gave the strongest strength of the extraction resulted in the weakest cationic response as shown in Table 11. In a sample solution of uranium sulfate, a membrane with phosphine oxide obtained better response and attained the same slope value as that of T B P for which both approximately gave the same order of extraction strength. The quantitative description for the effect of the equilibrium extraction properties upon the membrane potential properties has also been reported (24, 25). It concluded that the stronger the affinity of the ligand, the less the response of the membrane. Comparatively, use of stronger ligand reduced the membrane re-
-LO
-30
-2.0
-10
log ado2 Flgure 2. The potential responses of the membranes containing TBP TOPO sodium tetraphenylborate: (a) 400/100/20mg of TBP/ TOPO/sodium tetraphenylborate, (b) 400/50/50mg of TBP/TOPO/ sodium tetraphenylborate,(c-e) 400/100/50 mg of TBP/TOPO/sodium tetraphenylborate, (f) 400/-/50 mg of TBP/TOPO/sodium tetraphenylborate, (9) 4001-1100 mg of TBP/TOPO/sodium tetraphenylborate; (a-c) UO,(NOB)pbathing solution, (e-g) UOpS04bathing solution, (d) UOpCI, bathing solution.
+
+
sponse but probably increased the selectivity ratio. Selection of an appropriate organophosphorus ligand for uranium(VI) without incorporation of another compound to improve the membrane matrix seemed difficult. Use of T B P alone was considered even too strong a ligand for the uranium(V1) to establish a suitable response of the electrode in view of the resulting slope of 59 mV of membrane 10. In the case of TOPO, because of the high affinity and permeability to the anions, in the extreme case as the ligand a t the membrane surface approached saturation, namely, membrane exposed to the solution of high sample activities, the membrane gained full Nernstian anion responses. For this result, the mobility of the complexed uranium(V1) may be smaller than that of the coion. Since ideal cation permselectivity was necessary for the development of full Nernstian cationic responses, increasing the density of negative sites within the membrane matrix by introduction of lipophilic anions has given great improvements in the response characteristics of the neutral carrier liquid membranes (27). Incorporation of sodium tetraphenylborate was found effective as shown in Figure 2 and the optimum composition of TOPO/sodium tetraphenylborate was 100 mg/50 mg (membrane 10, Figure 2c-e). Without TOPO, membranes containing sodium tetraphenylborate alone did not yield an extensive detection range (membranes 12 and 13, Figure 2f,g). Increasing the amounts of sodium tetraphenylborate from 20 to 50 mg (membrane 9, 10) shifted the maximum in the response curves from the uranyl activities to 8.2 X M (Figure 2a,c), in agreement with about 3 X the demonstration given in ref 28. On introduction of sodium tetraphenylborate amounts greater than 100 mg, one obtained an unsatisfactory membrane, with loss of mechanical strength when in use. Among the membranes examined, membrane 10 was the most favorable one giving nearly the same electrochemical behavior in different uranyl salt solutions (Figure 2c-e), although the maximum response was still observed in the high activities of uranyl nitrate solutions. It showed monovalent cationic potential functions corresponding to the gained slope value of 59 f 1 mV, and an advantage of enhanced sensitivity was
2336 0 ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
I 1
2
3
L
PH
Flgure 3. pH effect on the potential responses of membrane IO: (a) IO-* M U02S04bathing solution, (b) M U0,S04 bathing solution, (c) dilute H2S04, uranium ( V I ) absent bathing solution.
Table 111. Selectivity Coefficients of the Membrane at Uranyl Level 1.0 x M (pH 3 ) interfering compd ion added as KilJJot Caz+ Ba2+ Ni2+ cuz+ Fe3+ ~ 1 3 +
Na' c1S0,Z'
Cr,0,2a
3.5 x 10-3 9.8 x 10-3 3.6 X 1.5 x 10-3 8.3 x 10-3 a 5.7 x 10-3 1.7 X 1.3 x 10-3 2.5 x 10-3 6.5 x 10-3
Bathing solution at pH 2.
expected therein. Monovalent response of uranyl ion selective electrode has been reported (10) and ascribed to the permeating species of the type U020He, in consideration of the most probable ionic species in the analyte. Although the evidence is not conclusive from the present work, the electrode indeed showed pH dependence as indicated in Figure 3. At pH below 1.5, membrane potential arises due to the contribution of hydrogen ions; the electrode responds to the hydrogen ions in the absence of uranium(V1) (Figure 3c). This is the result of the higher basicity of TOPO. Table 111gave the selectivity coefficients of the membrane in the uranyl nitrate bath solutions which contained varied amounts of interfering ions. The results indicated that only nickel(I1) gave slight interference. Electrode selectivity was related to the equilibrium ion exchange and mobility in the membrane (29).
where ub and ujsare the mobilities of cation i and j complexed with ligand s in the membrane and Ki and Kj are the partition coefficients of i and j between the outside solution and the respective complexes in the membrane. Ions in the PVC membrane were subject to restricted mobilities which were approximately the same for all counterions, especially when they were complexed with a long chain ligand (30, 31). Therefore, the selectivity was related simply to the exchange
equilibrium constant, Kj/Ki. Consequently the high selectivity for uranyl ions was expected in examining the extraction properties of TOPO from ref 22. The prepared membrane had good mechanical strength and had ohmic resistivities of about lo7 D cm. It could be stored for a long period in contrast to the other systems, since the PVC membrane contained only the mixture of ligands but no uranium. The changes of uranium concentrations in the internal solutions showed that it preferred the more dilute one. In low concentration like lo4 M, the electrode reproducibility, stability, and responses to the dilute analyte were much enhanced. The lifetimes of the electrode were about 1month, with a response slope of 50 mV/decade after 1 week. The potential stability was less than 40.1 mV for short term and less than f2.5 mV for long term. In conclusion, when the membrane was trapped with T B P and TOPO, the strong ligands together with their synergetic effect led it to respond to anions, particularly in uranyl nitrate. The incorporation of sodium tetraphenylborate can eliminate the anionic interference and improve the response in low sample concentrations but not at concentrations higher than 10-1 M. Direct introduction of the ligand and diluent for the preparation of the PVC membrane obtains satisfactory results. The previous synthesis of uranium organophosphorus complex is not necessary.
LITERATURE CITED Manning, D. L.; Stokely, J. R.; Magouyrk, D. W. Anal. Chem. 1974, 4 6 , 1116. Mlkhailov, V. A.; Oslpov, V. V.; Serebrennikova, N. V. Zh. Anal. Khlm. 1978, 3 3 , 1154. Goldberg, I.; Meyerstein, D. Anal. Chem. 1980, 5 2 , 2105. Freiser, H. "Ion-Selective Electrodes in Analytical Chemistry"; Plenum Press: New York, 1978; Chapter 4. James, H. J.; Carmack, G. P.; Frelser, H. Anal. Chem. 1972, 44, 853. Eyal, E.; Rechnitz, G. A. Anal. Chem. 1971, 4 3 , 1090. Durst, R. A., Ed. NBS Spec. Publ. 1969, No. 314, Chapter 1. Moody, G. J.; Thomas, J. D. R. "Selective Ion Sensitive Electrodes"; Merrow: Watford, 1971; Chapter 2. Srlnlvasan, K.; Rechnltz, G. A. Anal. Chem. 1969, 4 1 , 1203. Senkyr, J.; Ammann, D.; Meler, P. C.; Morf, W. E.; Pretsch. E.; Simon, W. Anal. Chem. 1879, 5 1 , 786. Freiser, H. "Ion-Selective Electrodes in Analytical Chemistry"; Plenum Press: New York, 1978; Chapter 3. Craggs, A.; Keil, L.; Moody, G. J.; Thomas, J. D. R. Talanfa 1975, 22, 907. Healy, T. V.; Kennedy, J.; Waind, G. M. J. Inorg. Nucl. Chem. 1959, 70, 137. Irving, H.; Edgington, D. N. J. Inorg. Nucl. Chem. 1960, 15, 158. Marcus, Y. Chem. Rev. 1963, 63, 139. Sato, T. J. Inorg. Nucl. Chem. 1965, 2 7 , 1853. Sato, T. J . Inorg. Nucl. Chem. 1963, 25, 109. Sato, T. J. Inorg. Nucl. Chem. 1962, 2 4 , 699. Baes, C. F.; Zlngaro, R. A.; Coleman, C. F. J. Phys. Chem. 1958, 6 2 , 129. Peppard, D. F.; Ferraro, J. R. J. Inorg. Nucl. Chem. 1959, 70,275. RuZiEka, J.; Hansen, E. H.; Tjell, J. C. Anal. Chlm. Acta 1973, 6 7 , 155. White, J. C.; Ross, W. J. Natl. Acad. Sci. Nucl. Sci. 1961, NAS-NS3102. Healy, T. V.; Kennedy, J. J. Inorg. Nucl. Chem. 1959, 10, 128. Clanl, S.;Eisenman, G.; Szabo, G. J. Membr. Biol. 1969, 1, 1. Boles, J. H.; Buck, R. P. Anal. Chem. 1973, 4 5 , 2057. Koryta, J. Anal. Chim. Acta 1972, 61, 329. Morf, W. E.; Kahr, G.; Simon, W. Anal. Lett. 1974, 7 , 9. Seto, H.; Jyo, A.; Ishlbashl, N. Chem. Lett. 1975, No. 5 , 483. Buck, R. P. Crlt. Rev. Anal. Chem. 1975, 5 , 323. Sandbiom, J.; Elsenman, 0.;Walker. J. L. J. Phys. Chem. 1967, 71, 3862. Elsenman, G.Anal. Chem. 1968, 4 0 , 310.
RECEIVED for review March 1, 1982. Accepted July 19, 1982.
